Optical transmitters and optical transceivers used in optical communications typically use laser diodes to generate optical signals that represent data, which are then transmitted over optical fibers. The laser diodes are typically integrated into laser diode ICs. In single-channel transmitters and transceivers, the laser diode IC typically contains a single laser diode. In multi-channel transmitters and transceivers, often referred to as parallel transmitters and parallel transceivers, the laser diode IC typically includes the same number of laser diodes as there are transmit channels in the transmitter or transceiver. The laser diodes are driven by laser diode driver circuitry. The laser diode driver circuitry is typically contained in a laser diode driver IC that is separate from the laser diode IC. The laser diode driver IC produces the electrical signals that drive the laser diode or diodes.
The laser diode IC, the laser diode driver IC and a controller IC are typically mounted on a circuit board of the transmitter or transceiver module. Typically, a monitor photodiode IC is also mounted on the circuit board. Other components may also be mounted on the circuit board, such as, for example, an analog-to-digital converter (ADC) and a digital-to-analog converter (DAC). Conductive output pads of the laser diode driver IC are wire bonded to conductive input pads of the laser diode IC. Conductive input pads of the laser diode driver IC are wire bonded to conductive output pads of the controller IC. Conductive output pads of the monitor photodiode IC are wire bonded to conductive input pads of the controller IC. The monitor photodiode IC includes one or more monitor photodiodes that monitor the light output power (LOP) level of one or more respective laser diodes of the laser diode IC. The monitor photodiodes output electrical signals related to the amount of light impinging thereon. These electrical signals are typically analog signals, which are converted into digital signals by an ADC and then fed back to the controller IC. The controller IC processes these digital signals and produces output signals that are output from the controller IC to the laser diode driver IC. These signals adjust the bias and/or modulation currents of the laser diode driver circuits such that the electrical drive signals that are output from the laser diode driver IC to the laser diode IC ensure that the laser diodes output the proper LOP levels for representing a logic 0 level and a logic 1 level and are biased at an operating condition that allows the laser diodes to have optical characteristics, such as rise-time and fall-time, that are appropriate for the particular application.
Degradation of the LOP level provides information about the health of the laser diode. For example, if a drop in the LOP level occurs that is not caused by a change in the bias or modulation currents of the laser diode, this may be an indication that the laser diode has failed or that failure of the laser diode is likely to occur soon. However, information in addition to, or in lieu of, LOP level information is needed to accurately determine the health of the laser diode.